Bipolar junction transistors (BJTs) are commonly used in analog integrated circuit (IC) devices. A BJT includes two p-n junctions sharing a cathode or anode region. The shared region is called the base. The base is a lightly doped region. The base separates two regions of opposite conductivity type: the emitter and the collector. Depending on the conductivity types, a BJT can be of the NPN variety or the PNP variety.
Supplying a current to the base region of a BJT can induce a current of much greater magnitude between the emitter and the collector. The ratio between the current supplied to the base and the resulting current between emitter and collector can be referred to the gain of a BJT. The current between emitter and collector is the result of bidirectional flow of two type of charge carriers. Gain is reduced by recombination of these different types of charge carriers within the base region.